1988 Mazda B2200 Air/Fuel Solenoid rapping

Air/Fuel Solenoid inside carburetor between the primary and secondary jets is rapping (opening and closing rapidly). I can hear it with the ignition on (motor not running) and my ear on the air filter canister.

I pulled the entire carburetor to rebuild and replaced all vacuum hoses. I cleaned the Solenoid and tested with battery and blowing air through it. Works like it's supposed to outside of the truck as well as all other solenoids on carburetor.

Wire jackets to A/F Solenoid were then wrapped in electrical tape to ensure no grounding.

I cleaned all grounds in engine compartment and firewall, but it still makes noise.

At time of carb rebuid, fuel/air screw was 1.25 turns out. After rebuild, I have it at 2 turns out and seem to get same MPG. Spark plugs don't indicate it running too rich.

Is this an indication of a bad ECU?

My next attempt is to bypass the wire harness and run the A/F Solenoid wires (2) directly to pins on ECU in the passenger cab.